The invention relates to compensating circuits of the type used to compensate for lack of flatness in the frequency response of signal transmission circuitry. The compensating circuits of the type in question exhibit a frequency response curve (a curve of gain or attenuation plotted against frequency) into which can be introduced elevations or depressions of selected magnitude at selected frequency ranges. Usually, it is possible to select both the middle frequency and bandwidth of each elevation or depression to be introduced into the frequency response curve of the compensating circuit and also the magnitude of the elevation or depression to be introduced into the curve. Although such compensating circuits are in general utilized to compensate for lack of flatness in the frequency response of a transmission path, they may in certain circumstances also be used to accentuate or deaccentuate the transmission of certain spectral components of transmitted signals, for example to suit the taste of the listener of a phonographic sound system.
Many compensating circuits of the general type in question are already known, for example in Federal Republic of Germany Pat. Nos. 828,258 and 828,715 and Auslegeschriften No. 1,261,553 and 1,267,261. However, in all of these known circuits, it is necessary to use either multicontact switches or else potentiometers for effecting at least one of the three requisite adjustments (i.e., of the middle frequency of the elevation or depression to be introduced into the frequency response curve, of the bandwidth of the elevation or depression to be introduced, and of the magnitude of the elevation or depression to be introduced). Multicontact switches and potentiometers are in general disadvantageous because they are very susceptible to malfunction and also can result in considerable power losses.
Federal Republic of Germany Auslegeschrift No. 2,135,204 discloses an arrangement for equalizing broad band communications signals. In that arrangement, adjustable compensating components of various types, such as pilot stages, reasonance circuits, echo compensators and switchable attenuation stages, are connected parallel to one another in the transmission path for the communications signals, intermediate a circuit branch point and a summing junction; a permanently wired circuit branch serving as the main transmission path is provided with attenuation stages and is connected in parallel to the compensating signal transmission paths. This known circuit in part avoids the aforementioned difficulties; what is involved really is an arrangement for connecting together a plurality of two-terminal-pair compensating stages. The differential capacitors employed for summing the several compensating signals which are generated are connected as high-impedance components to the outputs of the two-terminal-pair stages and do not form components of resonant circuits.
Additionally, this known arrangement includes at least one amplifier at the input to which the signal level is quite low, so that the signal-to-noise ratio of the compensating arrangement is not good.